1. Field of the Invention
The present invention relates to a valving device for controlling gaseous feed of organo-metallic compounds used typically in a metallorganic chemical vapor deposition (MOCVD) apparatus for making thin films of ferroelectric or highly dielectric materials.
2. Description of the Related Art
In recent years, there has been a quantum jump in circuit density of integrated circuit devices produced by the semiconductor industry, and intense development activities are underway in anticipation of giga-bit order DRAMs replacing the prevailing mega-bit order DRAMs of today. Dielectric thin film materials Ad used to make high capacitance devices necessary for producing DRAMs have, in the past, included silicon oxide or silicon nitride films of dielectric constant less than ten, tantalum pentaoxide (Ta.sub.2 O.sub.5) films of dielectric constant of about twenty; metal oxide films of dielectric constant of about three hundred, such as barium titanate (BaTiO.sub.3) or strontium titanate (SrTiO.sub.3) films, or barium/strontium titanate films of a mixed composition, appear to be promising. Promising also are even higher dielectric materials such as lead-zinc-titanate (PZT), lead-lithium-zinc-titanate (PLZT) and Y1.
Of the various methods for making such thin films, prospects are particularly bright for the metallorganic chemical vapor deposition (MOCVD) process, and in this case, it is necessary that a gaseous feed must be supplied in a steady gas stream to asubstratedisposed inafilmdepositionchamber. The gaseous feed is produced by heating and vaporizing a liquid mixture, produced by dissolving such materials as Ba(DPM).sub.2 or Sr(DPM).sub.2, which is solid at normal temperature into some organic solvent such as tetrahydrofuran or THF.
However, such gaseous materials present a serious problem that they can exist in thermodynamic stability only in a narrow range of pressures and temperatures. This means that if temperature drops or pressure rises in the system, solid components in the gaseous feed can deposit, and if the ambient temperature rises, reaction can take place to produce residual solid particles. Because of the narrow opening between a valve body and a valve seat, when the gaseous material is passing through a valve, adiabatic expansion causes a local drop in the valve temperature. Changes in temperature or pressure of the process gas cause deposition of residual particles on such valving devices as gas flow control valves or feed switching valves, ultimately resulting in degradation in the performance of the affected valving devices.